1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a liquid crystal display device and a method of fabricating a liquid crystal display device.
2. Description of the Related Art
In general, cathode ray tube (CRT) devices have been commonly used to display images. However, due to their size and weight limitations, the CRT devices are increasingly being replaced with liquid crystal display (LCD) devices that are small sized and lightweight, and have low profiles and low power consumption.
The LCD devices include an array substrate upon which thin film transistors (TFTs) are arranged, a color filter substrate upon which red, green, and blue color filter layers are formed and which is attached to the array substrate, and liquid crystal material interposed between the array and color filter substrates. The array and color filter substrates are formed by patterning and etching metal and insulating layers using photolithographic processes including several masking steps.
Fabrication of the array substrate includes a first mask step, wherein a metal layer is deposited onto a transparent glass substrate and then etched to form a gate bus line and a gate electrode. Next, during a second mask step, a gate insulating layer, an amorphous silicon film, and a doped amorphous silicon film are coated on the transparent glass substrate to form an active layer. Then, a third mask step includes depositing a source/drain metal film onto the glass substrate and patterning the metal film to form source/drain electrodes on the active layer and a data bus line. During a fourth mask step, a passivation film is deposited onto the glass substrate and a contact hole is formed in the passivation film. Then, during a fifth mask step, an ITO transparent film is deposited onto the substrate and etched to form a pixel electrode.
Since manufacturing costs are dependent upon the total number of masking steps used to fabricate the array substrate, significant consideration has been given to reduce the total number of masking steps. Accordingly, the third and fourth mask steps can be combined to reduce the total number of masking steps to four masking steps. In order to successfully perform the four masks process, a half-tone mask is used to concurrently form the source and drain electrodes and the active layer, wherein a photoresist film is patterned and etched using the half-tone mask. Alternatively, a slit-type mask having a slit pattern with a resolution less than a normal resolution is inserted for an etch in the exposure process of the photoresist film.
FIGS. 1 to 4 are cross sectional views of a method for fabricating a liquid crystal display device using a four masks process according to the related art. In FIG. 1, a metal film, such as aluminum (Al), chromium (Cr) or the like, is deposited on a transparent insulating substrate 10 using a sputtering method. Then, a photoresist film is coated onto the metal film and is exposed to light using a first mask to form a photoresist pattern. Next, the metal film is wet-etched using the photoresist pattern as an etch mask to form a gate electrode 1, a gate bus line 21, and a gate pad 11 on the transparent insulating substrate 10. In addition, the gate electrode 1, gate bus line 21, and gate pad 11 may include an additional conductive layer formed exclusively on upper surfaces of the gate electrode 1, gate bus line 21, and gate pad 11
In FIG. 2, a gate insulating layer 3, an amorphous silicon film (a-Si:H) 5, and an n+ doped amorphous silicon film 7 are sequentially deposited onto the transparent insulating substrate 10. Next, a metal film is deposited on the transparent insulating substrate 10. Then, a photoresist film is coated onto the metal film and is exposed and developed by using a second mask to form a photoresist pattern.
The second mask includes a slit such that a half-tone pattern is used to form a channel layer region of the amorphous silicon film (a-Si:H) 5. Accordingly, the amount of light irradiated onto the photoresist film through the slit of the second mask is decreased. Thus, the region of the photoresist film corresponding to the region of the slit of the second mask receives only a portion of the irradiated light.
Then, an etch process is performed to simultaneously form the source electrode 9a, the drain electrode 9b, the data bus line, the data pad 31, the ohmic contact layer 7, and the channel layer 5. In other words, a single masking process enables simultaneously formation of the source electrode 9a, the drain electrode 9b, the data bus line, the data pad 31, the ohmic contact layer 7, and the channel layer 5, thereby decreasing the total number of mask steps.
In FIG. 3, a passivation film 13 is formed on the transparent insulating substrate 10. Then, contact holes are formed in the passivation film 13 using a third mask step to expose the gate pad 11 and the data pad 31.
In FIG. 4, a transparent conductive film of ITO is deposited along an entire surface of the transparent insulating substrate 10. Then, a photoresist film is coated onto the ITO film and patterned by using a fourth mask to form the pixel electrode 15, a gate pad pattern 25, and a data pad pattern 27. The pixel electrode 15 is formed to overlap the gate bus line 21 to form an auxiliary capacitance with the gate bus line 21.
However, as detailed above, manufacturing costs of the LCD devices are considerably higher using a four mask fabrication process-than costs associated with a three mask fabrication process. Accordingly, an LCD array substrate fabricated using a three masks process would result in lower costs and increase an overall time to fabricate the LCD array substrate. In addition, since large-sized LCD devices require increasing the lengths of the gate and data bus lines, low resistance wirings are required.